you will need to purchase a full EFI conversion kit. It will be very pricey... This includes the pump, manifold, injectors, pretty much everything.
Shop around and find one that is compatible with the 85 mustang. Or just try upgrading your carburetor. It will be a fraction of the cost and will deliver good results. While your at it, you could install a new cam and lifters and new cylinder heads and it would still be way cheaper than a full EFI conversion kit.
Ain't worth it. You would be money and performance ahead to trade it off for a newer one with factory EFI. You will not likely ever get it to run right in hot weather and cold, idling and full throttle, The electronics and the associated sensors, OMG. Good luck in getting it to pass a state emissions tests. Bad idea. Some people dumb down a highly modified EFI engine back to carburetors to get them to run right. You would need expert level understanding to pull it off, and then it would not be cheap.,
a fat wallet, and true love for the car also. You may need to get a computer, some chips, you may end up doin a million mods to the engine itself in order for it to be able to accept the fuel injection. Dude you are looking at some work. you may as well get an engine that is already set up for EFI, or DPI, to save some time, and possibly money. this may be near impossible.